1. Field of the Invention
The present invention relates to a method of manufacturing a vitreous silica crucible using a rotary mold method, which is capable of uniformly heating an inner surface of the crucible, and more particularly, a method of manufacturing a vitreous silica crucible, which is capable of uniformly forming the thickness of a transparent layer of a curved portion and a bottom from a straight body portion of the crucible by uniformly heating an inner surface of the crucible.
Priority is claimed on Japanese Patent Application No. 2007-310193, filed Nov. 30, 2007, the content of which is incorporated herein by reference.
2. Description of the Related Art
As a method of manufacturing a vitreous silica crucible used in pulling of single-crystal silicon, a rotary mold method is known. This method is a method of depositing silica powder on an inner surface of a rotating hollow mold to a predetermined layer thickness, melting the silica powder by arc discharge of electrodes provided above the mold, sucking and decompressing (evacuating) the inside of the deposition layer of the silica powder during arc melting, vitrifying the silica powder while de-airing the air therein, and forming the inner surface of the mold into a form of a crucible (Japanese Patent No. 1257513 (JP-B No. 59-34659)).
As shown in FIG. 8, in the conventional manufacturing method, a plurality of electrodes 12 is symmetrically (point-symmetrically or linear-symmetrically) provided with respect to a central axis line of the rotary mold, and the silica powder 11 at the position corresponding to the inner surface of the crucible is melted by center arc generated in the vicinity of the central axis line L of the mold. In the arrangement of the electrode shown in FIG. 8, high-temperature gas generated at the front end of the electrodes 12 symmetrically flows with respect to the central axis line Las shown in the right and left sides of FIG. 8, a portion thereof becomes outer circulating flow Gout flowing out along the inner surface of the mold and the residual high-temperature gas becomes inner circulating flow Gin which circulates in the inner space of the mold at the both sides of the electrodes.
The inner circulating flow generated in the center arc shown in FIG. 8 is reheated when the high-temperature gas flows along the electrodes, heat efficiency is excellent, but a temperature difference between the inner circulating flow and the outer circulating flow in the inner space of the mold occurs, and thus a portion which is excessively heated by the inner circulating flow is collapsed by a viscosity drop.
The present invention provides a method of manufacturing a vitreous silica crucible, which is capable of uniformly heating an inner surface of the crucible, and uniformly forming the thickness of a transparent Layer of a curved portion and a bottom from a straight body portion of the crucible, in order to solve excessive heating due to inner circulating flow in the conventional center arc.